[llvm] [RISCV][CostModel] Remove inst cost of cmp inst in cmp-select sequence. (PR #91158)

[Shih-Po Hung via llvm-commits]

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@@ -1436,6 +1436,13 @@ InstructionCost RISCVTTIImpl::getCmpSelInstrCost(unsigned Opcode, Type *ValTy,
     }
   }
 
+  // The cmp + select instructions will lower to SELECT_CC and lower to
+  // PseudoCCMOVGPR which will generate a conditional branch + mv. The
+  // cost of (cmp + select) will be (0 + select inst cost).
+  if (I && I->hasOneUser() && isa<SelectInst>(I->user_back()) &&
+      I->user_back()->getOperand(0) == I)
+    return 0;
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arcbbb wrote:

We need to verify whether `I` is an ICmp instruction.

https://github.com/llvm/llvm-project/pull/91158